Abstract: In practical engineering application, when mechanical structural parts are subjected to repeated loads, the internal stress and strain in the metal are often asymmetric. Under asymmetric cyclic loading condition, the metal does not only exhibit cyclic soft/hardening characteristics, but also exhibits average stress relaxation phenomenon. This phenomenon affects its mechanical properties in the stable cycle, and then affect the strength and safety of the mechanical structure under corresponding working conditions. For the GS-20Mn5 steel commonly used in the critical structures of heavy presses, monotonic tensile test and asymmetric strain cyclic loading tests with a strain ratio R of 0.5 and strain amplitudes of 0.20%, 0.25%, 0.27%, 0.30% and 0.40% are conducted, and the corresponding hysteresis curves are obtained. For the monotonic tensile condition, A-F kinematic hardening model is established. For the asymmetric cyclic loading, a modified Landgraf model is constructed to describe its average stress relaxation, and it is applied to the Ramberg-Osgood formula, combined with the A-F nonlinear kinematic hardening model, the material constitutive model corresponding to the cyclic stress-strain curve under asymmetric cyclic loading condition is established. Then the constitutive model parameters are determined. For a upper beam of a heavy forging hydraulic press under asymmetric cyclic load, the established constitutive models are used in its numerical shakedown analysis and the upper beam's strength safety under cyclic loading is evaluated. The result show that the shakedown limit load of the upper beam is increased by about 7% by adopting the asymmetric cyclic stress-strain constitutive model compared with adopting the monotonic tensile model.

Key words: asymmetric cyclic loading, mean stress relaxation, cyclic softening/hardening, constitutive model, upper beam of large press